Zn-induced layer exchange of p- and n-type nanocrystalline SiGe layers for flexible thermoelectrics

Fermi-level control in a polycrystalline SiGe layer is challenging, especially under a low thermal budget owing to the low activation rate of impurities and defect-induced acceptors. Here, we demonstrate the low-temperature (120–350 °C) synthesis of nanocrystalline p- and n-type Si_1−xGe_x (x: 0–1) layers using the layer exchange technique with a Zn catalyst. Pure Zn formed p-type SiGe layers (hole concentration: 10²⁰ cm⁻³ for x ≥ 0.8) due to the shallow acceptor level of Zn in Ge. Conversely, As-doped Zn allowed us to synthesize n-type SiGe layers (electron concentration: 10¹⁹ cm⁻³ for x ≤ 0.3) at the lowest ever temperature of 350 °C, owing to the self-organized As doping to SiGe during layer exchange. The resulting p-type Si_0.2Ge_0.8 and n-type Si_0.85Ge_0.15 layers exhibited the largest ever power factors (280 μW/mK² for the p-type and 15 μW/mK² for the n-type), for SiGe fabricated on a flexible plastic sheet. The low-temperature synthesis technology, for both p- and n-type SiGe layers, opens up the possibility of developing human-friendly, highly reliable, flexible devices including thermoelectric sheets.